1. Field of the Invention
The invention relates to a device for contacting a welding wire in a welding torch, comprising at least two contact shells having a contact area for contacting the welding wire.
The invention further relates to a contact shell for contacting a welding wire in a welding torch, said contact shell having a contact area for the welding wire at one end thereof.
Finally, the invention relates to a method for contacting a welding wire in a welding torch, wherein the welding wire is contacted in a contact area of at least two contact shells.
The present invention relates to the contacting of a welding wire in a welding torch with a constant contact force at a defined contact point. This results in a constant transfer of current to the welding wire, so that a constant welding quality may be guaranteed. For achieving a constant contact force, the factors described in the following have to be taken into account. The substantial factors are the tolerance of the diameter of the welding wire, dirt, and abrasion. This means in detail that the diameter is not exactly constant along the length of the welding wire. Likewise, abrasion is produced in the wire core due to the conveyance of the welding wire, said abrasion also being conveyed along to the contact places. Additionally, dirt collects on the coil onto which the welding wire is wound, said dirt also being conveyed to the contact points. In addition, the coil causes a certain deformation of the welding wire, the so-called cast. Likewise, the contact points are ground during the conveyance of the welding wire. These factors cause a variation of the diameter of the welding wire. Thus, the contact points have to be correspondingly variable to be able to ensure the desired constant contact force.
2. Description of the Related Art
WO 2008/018594 A1 or JP 2002059265 A disclose devices for contacting a welding wire. These devices exert, via a spring, a pressure on a movable contact member that is consequently pressed against a cone of a protective sleeve, and the opening arranged in the contact member is correspondingly adapted to the welding wire, and the welding wire is contacted with a wire contact force. Thus, an axial force of the spring is converted into a force acting radially on the welding wire, the wire contact force.
It is of disadvantage that the initially described factors are not taken into account with such constructions of a contacting device. As already mentioned, for an optimum and low-wear contacting during the welding process, the movable contact member permanently has to adapt to the welding wire with a defined wire contact force. This adaptation has to be effected due to a minimal movement of the contact members which has to be enabled by the contacting system. In prior art, the following disadvantages, however, result in this respect.
Any movement of the contact members results in differently directed friction forces at the cone of the protective sleeve, which are, due to the characteristic of the device, in the order of magnitude of the spring force itself that constitutes the largest force. The reason for this is that the contact members have to move against the spring force so as to adapt to the welding wire. Thus, the friction forces change and an actual wire contact force results which deviates strongly from the defined wire contact force and hence is no longer optimum. This means, however, also that the friction forces and the wire contact forces are dependent on each other.
Likewise, a contact force between the contact member and the cone arises due to the construction. Thus, the cone effects both the contact force and the wire contact force, and hence the wire contact force additionally also depends on the contact force. Thus, for the defined wire contact force there results a contact force ranging in the order of magnitude of the wire contact force. The contact force should, however, be substantially higher than the wire contact force since, in addition to the electric current, the contact heat should also be transferred to the protective sleeve there so as to cool the contact member as good as possible.
A change of the wire conveying direction which takes place in the case of special processes also has a great influence on the wire contact forces arising, since it is not possible to react to such quick changes of movement.
It is also disadvantageous that the contact member, due to the construction thereof, always guides the welding wire centrically. Thus, it is not possible for the welding wire to evade in any direction. Likewise, the welding wire is not just contacted by the contact member, but also guided by same. This causes that possible lateral guide forces due to wire cast and torch angle also have an influence on the wire contact forces, since the contact members are ground more quickly.
In summary, there may be said that, due to the dependency of the forces, the wire contact force required for the constant contacting changes with any influence.
It is also disadvantageous with those prior art designs in which the contact member is screwed that the contact force on the welding wire cannot be readjusted automatically, since the contact member does not have the required flexibility.